Crystal-Melt Partition Coefficients of Impurities in Forsterite, Mg2SiO4: Experimental Determination, Crystal-Chemical Analysis, and Thermodynamic Evaluation

• Published in: Inorganic materials 2005-06, Vol.41 (6), p.627-638
• Main Authors: Dudnikova, V. B., Urusov, V. S., Zharikov, E. V.
• Format: Article
• Language: English
• ISSN: 0020-1685; 1608-3172
• DOI: 10.1007/s10789-005-0181-2

The forsterite-melt partition coefficients K are determined experimentally for a large number of mono-, di-, tri-, and tetravalent impurities. The energies of native defects and impurities (E(d)) and the solution energies (E(s)) of impurities in forsterite are evaluated using computer simulation. The defect energy is shown to vary linearly with the difference in ionic radius between the host and substituent atoms (Deltar) and with the impurity cation charge, while the partition coefficient and solution energy of impurities are quadratic functions of these parameters. The plots of 1nK versus (Deltar)2 and E(s) versus (Deltar)2 for isovalent substitutions (Me(x)(Mg) and Me(x)(Si)) pass close to the origin, in contrast to the plots for heterovalent substitutions (Me(Mg) and Me x (Si)). The significant y intercept of the latter plots is interpreted as evidence for the formation of extra defects maintaining electroneutrality. The y intercept of the plot of E(s) versus (Deltar)2 is 2 eV, which is about half the formation energy of Frenkel defects in forsterite. The best fit equations representing the correlation between the partition coefficients and solution energies of impurities demonstrate that heterovalent substitutions increase the entropy contribution to the free energy of solution of impurities.